1. Cell Membrane and Transport
Recapitulation

2. Learning Outcomes
(g) Describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins.
(h) Outline the roles and functions of membranes within cells and at the surface of cells. (Knowledge of osmosis, facilitated diffusion, active transport, endocytosis and exocytosis is required.)

3. Learning Outcomes
(g) Describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins.
(h) Outline the roles and functions of membranes within cells and at the surface of cells. (Knowledge of osmosis, facilitated diffusion, active transport, endocytosis and exocytosis is required.)

4. Fluid Mosaic Model
In the fluid mosaic model of the cell membrane, the membrane is visualised as a fluid structure of a phospholipid bilayer embedded or attached with a mosaic of various proteins.

5. Components of Membrane
Phospholipids
Cholesterol
Glycolipids
Proteins
Glycoproteins

6. Phospholipids

7. Phospholipids
Phospholipids are amphipathic (have both hydrophobic and hydrophilic regions)
In aqueous surroundings, the hydrophilic phosphate heads face the water while the hydrophobic hydrocarbon tails aggregate away from the water, forming a phospholipid bilayer with a hydrophobic core

8. Phospholipids and fluidity
Temperature increases, kinetic energy of phospholipids increases, fluidity of membrane increases  fluid state
Temperature decreases, kinetic energy decreases, fluidity decreases  semisolid state
Longer hydrocarbon chains, more extensive hydrophobic interactions between phospholipids, fluidity decreases
More unsaturated hydrocarbon tails, more kinks, tails unable to pack closely, fluidity increases
More saturated hydrocarbon tails, more straight chains, tails able to pack closely, fluidity decreases
Note the flow of thought in reasoning

9. Cholesterol

10. Cholesterol’s effects
Found only in animal cell membranes
Intercalated into both layers  enhance mechanical stability of membrane
Dual effect on membrane fluidity
High temp, interferes with phospholipid movement, reduces membrane fluidity
Low temp, prevents close packing of phospholipids, increases membrane fluidity
Wedged between phospholipids, fills in spaces between them, prevents passage of small polar molecules and ions, decreases membrane permeability

11. Proteins
Schematic representation of transmembrane proteins: 1. a single transmembrane α-helix (bitopic membrane protein) 2. a polytopic α-helical protein 3. a transmembrane β barrel The membrane is represented in light brown.
Schematic representation of the different types of interaction between monotopic membrane proteins and the cell membrane: 1. interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix) 2. interaction by a hydrophic loop 3. interaction by a covalently bound membrane lipid (lipidation) 4. electrostatic or ionic interactions with membrane lipids (e.g. through a calcium ion)

12. Functions of membrane proteins
Anchorage
Recognition (usually glycoproteins)
Enzymes – catalyse reactions
Receptor – cell signalling
Carrier – facilitated diffusion or active transport
Channels – leak/gated channels (facilitated diffusion)
Intercellular joining

13. Glycolipids Consist of lipids attached to oligosaccharide chains
Carbohydrate groups are highly hydrophilic
Occur on the side of the cell membrane facing the extracellular fluid
Function to bind extracellular signal molecules, intercellular adhesion and cell-to-cell recognition

14. Glycoproteins
Consists of proteins with oligosaccharide chains attached; the short carbohydrate chain is attached during post-translational modification of the protein
Found in the cell membrane and cytosol. Often occurs as integral proteins
Variety of functions, including as a cell attachment recognition site and receptor

15. Learning Outcomes
(g) Describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins.
(h) Outline the roles and functions of membranes within cells and at the surface of cells. (Knowledge of osmosis, facilitated diffusion, active transport, endocytosis and exocytosis is required.)

16. Roles and functions of membranes
Within Cell
Compartmentalization – provide suitable environment (pH, localised molecules) for reactions to occur
At Cell Surface
Define cell’s boundaries
Regulate transport of substances in and out of cell (partially permeable membrane)
Cell-to-cell communication (using receptor proteins, etc.)
Localisation of structures

17. Transport across membranes
Major function of cell membrane: to regulate entry and exit of substances to and from cell
Regulated by components of cell membrane
Phospholipids (and cholesterol)
Proteins
Channel protein (provides pore)
Carrier protein (changes conformation to carry substances across)

18. Transport across membranes

19. Osmosis
The net movement of freely moving water molecules from a region of less negative water potential to a region of more negative water potential, through a selectively permeable membrane
Highest water potential is 0 (pure water)
Solutes reduce water potential because hydration shells are formed around solute particles, reducing the number of free water molecules
Animal cells: Water potential = solute potential
Plant cells: Water potential = solute potential + pressure potential
Pressure due to cell wall
Pressure potential is positive (pushes water out)

20. Effect of osmosis on cells

21. Simple Diffusion
Diffusion is the net movement of a substance from a region of higher concentration to a region of lower concentration, i.e. down a concentration gradient
Occurs for small non-polar molecules. See figure 11-1 of notes.
Hydrophobic molecules diffuse directly across the hydrophobic core of the phospholipid bilayer
No energy from ATP is needed
No carrier proteins are needed

22. Facilitated Diffusion
For larger, hydrophilic substances (ions, polar)
Facilitated by a transport protein in the membrane, without which the substance would not be able to cross the hydrophobic core of the phospholipid bilayer
Carrier protein (undergoes conformational change to transport substance)
Channel protein (central hydrophilic pore)
No ATP required
Down a concentration gradient

23. Active transport
The movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient
ATP required
Primary active transport
carrier protein-mediated
Occurs for ions and small hydrophilic molecules
Bulk transport
Involves invagination of the plasma membrane or the extension of pseudopodia
Occurs for large objects or macromolecules

24. Endocytosis

25. Exocytosis